Refractory composite articles

ABSTRACT

A porous substrate formed from an agglomeration of fibers preferably has a carbonaceous composition. A binder is applied to the substrate. The binder may illustratively be an acrylic or beeswax to vaporize and form pyrolized carbon during pyrolysis or may illustratively be a phenolic resin, a furfural resin or pitch to form pyrolyzed carbon on the fibers during pyrolysis. A mixture of particles having the same approximate size as the diameter of the fibers is then applied to the fibers to become mechanically locked in the substrate pores. The mixture may contain (a) a first material (preferably a compound containing boron) which oxidizes and melts at a relatively low temperature and is relatively viscous when melted, (b) a second material (preferably a compound containing silicon) which oxidizes and has an increased melting temperature and is more viscous than the first material when melted and (c) a refractory material (preferably a compound containing zirconium) having a considerably higher melting temperature oxide than the first and second materials. The first material may be obtained from group 3a of the periodic table, the second material from group 4a of the periodic table and the third material from groups 4b and 5b of the periodic table. An impermeable coating such as a carbide, oxide or a nitride is thereafter applied to the substrate. The impermeable coating tends to crack at elevated temperatures. The adjacent particles in the mixture melt and flow into the cracks to inhibit crack amplification. The ease of flow is dependent upon the proportions of the first, second and third materials in the mixture. The particles displaced in the mixture from the cracks remain mechanically locked to the substrate.

This invention relates to refractory composite articles. Moreparticularly, the invention relates to articles which are made fromrefractory composite materials and which are capable of operatingsatisfactorily under severe conditions in ranges of temperatures ofthousands of degrees fahrenheit. The invention further relates tomethods of producing such articles.

Various needs exist for materials which are capable of operatingsatisfactorily through elevated ranges of temperatures under severeoperating conditions. For example, jet engines now operate attemperatures of several thousands of degrees fahrenheit. The jet enginesare expected to operate at these temperatures for extended periods oftime without maintenance or overhaul. Rockets operate at even moreelevated temperatures than jet engines but their operative life span isconsiderably shorter than that of jet engines.

Materials are needed in such apparatus as jet engines and rockets forsuch applications as engine cowlings, engine housings and rockethousings. Such material should be light in weight to increase thepayload of the aircraft powered by the jet engines or to increase thethrust of the rockets. The material should also be capable ofwithstanding corrosion and erosion, particularly since the reactioncomponents of the fuel consumed in the engines or the rockets includescomponents which corrode or erode many materials. The material shouldalso be hard, tough and wear-resistant, particularly at the elevatedtemperatures which the material encounters in the jet engines orrockets.

Materials have been developed in the prior art, and are being used,which meet the objectives specified above on a somewhat limited basis.For example, materials have been developed which constitute acarbonaceous, porous substrate and which have layers deposited on thesubstrate to form a composite material which is somewhat hard, tough,light in weight, corrosion-resistant, erosion-resistant andwear-resistant. However, the material has had only limited success inall of these areas.

In view of the expanding need for such materials, a considerable efforthas been made, and a considerable amount of money has been expended, toexpand the parameters which refractory composite materials such as thosespecified above are able to accomplish. Such efforts have had only alimited success. The refractory composite materials capable of beingconstructed at present do not have the toughness, lightness, temperaturerange, wear-resistance, corrosion-resistance and erosion-resistance thatare desired. This has limited the effectiveness of products such as jetengines and rockets.

This invention provides a material which significantly extends the rangeof capabilities of refractory composite materials. The material isconsiderably more tough, hard, wear-resistant, corrosion-resistant anderosion-resistant than the materials of its class in the prior art. Thematerial is also effective in operation through a higher temperaturerange than the materials of its class in the prior art.

A porous substrate is formed from fibers having a carbonaceouscomposition and a diameter in the order of seven (7) or eight (8)microns. The fibers occupy approximately 40%-60% of the substrate space.A binder applied to the substrate is cured and then heated to pyrolyzethe binder. The binder occupies to 20%-40% of the substrate space. Thebinder may illustratively be an acrylic or beeswax to vaporize and formpyrolized carbon during pyrolysis or may illustratively be a phenolicresin, a furfural resin or pitch to form pyrolyzed carbon on the fibersduring pyrolysis.

A mixture of particles having the same approximate size as the thicknessof the fibers is applied to the fibers as a part of the agglomerate tobecome mechanically locked in the substrate pores. The mixture maycontain (a) a first material (preferably a compound containing boron)which oxidizes and melts at a relatively low temperature and isrelatively viscous when melted, (b) a second material (preferably acompound containing silicon) which has an oxide with an increasedmelting temperature and is more viscous than the first material whenmelted and (c) a refractory material (preferably a compound containingzirconium) having a considerably higher melting temperature as an oxidethan the first and second materials. The first material may be obtainedfrom group 3a of the periodic table, the second material from group 4aof the periodic table, and the third material from groups 4b and 5b ofthe periodic table.

An impermeable coating such as a carbide, oxide or a nitride isthereafter applied to the substrate. The impermeable coating tends tocrack at elevated temperatures. The adjacent particles in the mixtureoxidize and then melt and flow into the cracks to inhibit crackamplification. The ease of flow is dependent upon the proportions of thefirst, second and third materials in the mixture. The particlesdisplaced in the mixture from the cracks remain mechanically locked tothe substrate.

In the drawings:

FIG. 1 is a perspective view schematically illustrating a substrateformed from a plurality of fibers and a binder sheathing the fibers inthe substrate;

FIG. 2 is a perspective view schematically illustrating a substrateformed from a plurality of fibers and a binder deposited in theinterstices defined by the fibers;

FIG. 3 is a perspective view schematically illustrating one embodimentof an article formed from a refractory composite material includedwithin this invention; and

FIG. 4 is a perspective view schematically illustrating a secondembodiment of an article formed from a refractory composite materialincluded within this invention.

In one embodiment of the invention, a substrate generally indicated at10 is provided with porous characteristics. The substrate 10 may beformed from discrete elements such as fibers 12 in a woven or non-wovenrelationship. When the substrate 10 is non-woven, it may be assembled,braided, knitted or chopped. Preferably the fibers 12 are carbonaceoussuch as a fabric. For example, the substrate 10 may be formed frompyrolyzed wool, rayon, polyacrylonitride and pitch. The fibers may havea thickness of approximately seven (7) or eight (8) microns. However,the fibers 12 may also be formed from a material having a suitable basesuch as a ceramic base. Although the use of fibers as the discreteelements is preferred, the discrete elements may also constitute grainsor particles or needles.

As will be appreciated, the characteristics of the substrate 10 can beconsiderably varied without departing from the scope of the invention.For example, when the fibers 12 are used in a woven form, the relativenumbers of threads in warp and woof can be varied in great proportionsand the spacings between the threads can also be varied considerably.

The substrate 10 may be shaped to any desired configuration. A binder 14may then be deposited on the fibers 12 in the substrate 10. The bindermay sheath the fibers 12 as illustrated at 15 in FIG. 1. Instead ofsheathing the fibers 12, the binder 14 may fill the interstices betweenthe fibers 12, as illustrated at 18 in FIG. 2. The binder positions thefibers 12 in the substrate 10 and strengthens the substrate, at leastfor handling purposes. The binder may also incorporate particulates insuspension or the particulates may subsequently be added. The binder 14may then be pyrolyzed to form a layer of pyrolyzed carbon 16 on thefibers of the substrate 10. The layer of pyrolyzed carbon 16 is porousand permeable.

The substrate 10 can be coated with any suitable and conventional typeof resin such as phenolic designated by Monsanto Chemical Company asMonsanto 1008 resin to form the binder 14. As a first step, the phenolicmay be dissolved in a solvent such that it is fluid. The solvent mayillustratively be methylethylketone. The substrate 10 may then be coatedwith the phenolic and partially polymerized so that it is dry to thetouch. Other materials such as a furfural resin or pitch may also beused.

Patterns of the desired shape can then be cut from the substrate 10 orthe substrate can be heated to a temperature of at least 160° F.,compressed and then cut to the desired shape. The patterns in thedesired shape may then be placed in a mold and heated to a suitabletemperature such as approximately 350° F. to cure the phenolic. Thepatterns may then be cured at a temperature above 350° F. to stabilizethe system.

The phenolic is then pyrolyzed at a temperature of approximately 1600°F. to decompose it to carbon, its elemental form. One reason for thepyrolysis is to prevent the substrate from thereafter emitting gaseswhen it is heated. However, the discrete elements 12 are retained inposition by the pyrolyzed carbon 16 which may be disposed in theinterstices between the discrete elements or which may sheath thefibers, as shown in FIG. 3.

When the binder constituting the phenolic is pyrolyzed as discussed inthe previous paragraph, it loses approximately half of its mass and itshrinks and cracks. In spite of this, the pyrolyzed carbon is able tohold the discrete elements 12 in position in the substrate and to impartstrength to the substrate. The substrate 10 is then heated to atemperature of approximately 2200° F. to 4100° F. for a relatively shortperiod of time to impart a final shrink and pyrolyzation to the binder.

To provide the pyrolyzed carbon 16 on the substrate 10, the substratemay also be disposed in a furnace of conventional design. Thetemperature of the substrate may then be raised to a suitabletemperature such as approximately 1800° F., and a vacuum in the order of15 mm mercury (hg) may be maintained within the deposition chamber.Pyrolyzed carbon may then be deposited about each of the fibers 12 inthe substrate 10 as by flowing methane gas through the porous substratefor an extended period of time such as about fifty (50) hours.

Instead of using a wet binder or a gas to coat the discrete elements 12,a dry binder may be used. Dow Corning Polycarbosilane 6-2330 material isillustrative. This is a dry resin which may be mixed withmethylethylketone to provide a fluid mixture with which to work. Thismaterial has properties of becoming partially polymerized at roomtemperatures. The material is mixed with the substrate and is thenheated to a temperature of approximately 1600° F. to pyrolyze thebinder. The material is subsequently heated to a temperature of 2200° F.to 4100° F. to provide it with a final pyrolyzation and shrink.

Sodium silicate or colloidal alumina can also be used as binders. Thesubstrate 10 may be dipped into sodium silicate in liquid form and maythen be heated. The sodium silicate flows along the lengths of thediscrete elements 10 and tends to congregate or agglomerate at theinterstices between the discrete elements. The formation of layers ofmaterial at the interstices between the fibers of the substrate 10 isshown in FIG. 2.

The pyrolyzed carbon 16 facilitates the promotion of a good loadtransfer between the fibers 12, particularly when an article from therefractory composite material of this invention is stressed. Thepyrolyzed carbon 16 also helps to strengthen and stabilize the substrate10, particularly when the article is subjected to stress. It alsoenhances the mechanical interface for increasing the surface fractureenergy of the composite structure defining an article generallyindicated at 21 and formed from the refractory composite material ofthis invention. As a result, the pyrolyzed carbon 14 enhances thetoughness and flow resistance of the refractory composite article 21.

Instead of applying a material on the substrate 10 to retain pyrolyzedcarbon on the substrate after pyrolyzation, a material may be applied tothe substrate to vaporize during pyrolysis. The function of thismaterial is to retain the substrate as a composite material in a desiredshape as the substrate and the binder are cured. Such a binder mayconstitute an acrylic or beeswax.

A mixture of particles 20 is then applied to the substrate 10 or haspreviously been incorporated into the binder. Preferably the particleshave a size in the same order of magnitude as the diameter of the fibers12 in the substrate 10. For example, the particles 20 in the mixture mayhave a size of at least five (5) microns when the fibers 12 have athickness of approximately seven (7) or eight (8) microns. By providingparticles of this size, the particles become mechanically locked in thepores of the substrate 10. This mechanical locking tends to strengthenthe composite material which is produced.

Instead of being defined by particles, the mixture can be defined byneedles or a mixture of needles and particles. When needles areprovided, the mechanical locking of the mixture to the substrate isenhanced. However, as used throughout the specification and in theclaims, the term "particles" is intended to include needles and otherforms of discrete elements.

The particles 20 can be applied to the pores of the substrate 10separately after the pyrolysis of the binder or in combination with theapplication of the binder 14. The particles 20 can even be applied tothe pores of the substrate 10 before the application of the binder tothe substrate. Preferably the particles 20 are applied to the substrate10 either after, or in conjunction with, the application of the binderto the substrate and the pyrolysis of the binder.

The mixture of the particles 20 is preferably formed from a combinationof different materials. Preferably the mixture contains compounds ofboron, silicon and a refractory material such as zirconium. The boronmay be in the form of a boride such as zirconium boride (ZrB₂) or boroncarbide (B₄ C). The silicon may be in the form of a silicide or may bein some other form such as silicon carbide. The zirconium may be in theform of zirconium boride (ZrB₂) or in any other suitable form.

Other materials may be substituted for boron in the mixture of theparticles 20. For example, other elements in group 3a of the periodictable may be substituted for boron. Similarly, other elements in group4a of the periodic table may be substituted for silicon. Other elementsin groups 4b and 5b of the periodic table may be substituted forzirconium as a refractory material in the mixture. These includeniobium, hafnium, tantalum and titanium.

The relative percentages of the different materials in the mixture ofthe particles 20 are dependent upon the use which is to be made of thecomposite material constituting this invention. For example, when thecomposite material is used to sheath a jet engine, it may be subjectedto temperatures as high as 4000° F. and may be expected to withstandsuch temperatures for long periods of time while maintaining itsproperties for such long periods of time. In contrast, the compositematerial may be used in rockets or missiles. In such applications, thecomposite material may be subjected to temperatures as high as 5500° F.Under such conditions, the composite material may not have to maintainits properties for extended periods of time.

For applications at temperatures to approximately 2500° F., the mixtureof particles may have the following range of compositions:

    ______________________________________                                        Material Containing                                                                          Relative Amount By Volume                                      ______________________________________                                        Boron          65 to 95                                                       Silicon        1 to 21                                                        Refractory Material                                                                          1 to 21                                                        ______________________________________                                    

Preferably the mixture of the particles 20 has the following compositionfor applications at temperatures to approximately 2500° F.:

    ______________________________________                                        Material Containing                                                                          Relative Amount By Volume                                      ______________________________________                                        Boron          80                                                             Silicon        11                                                             Refractory Material                                                                          11                                                             ______________________________________                                    

When the composite material is to be used at temperatures aboveapproximately 2500° F., the mixture of the particles 20 may have thefollowing range of compositions:

    ______________________________________                                        Material Containing                                                                          Relative Amount By Volume                                      ______________________________________                                        Boron          55 to 85                                                       Silicon        5 to 25                                                        Refractory Material                                                                          7 to 27                                                        ______________________________________                                    

Preferably the mixture of the particles 20 has the following compositionwhen the compsoite material is to be used at temperatures aboveapproximately 2500° F.:

    ______________________________________                                        Material Containing                                                                          Relative Amount By Volume                                      ______________________________________                                        Boron          70                                                             Silicon        15                                                             Refractory Material                                                                          17                                                             ______________________________________                                    

An impervious coating 22 may be deposited on the substrate as bychemical vapor deposition. The impervious coating 22 may be formed froma carbide such as silicon carbide, titanium carbide or hafnium carbideor may be formed from a nitride such as silicon nitride. However, othermaterials may be used. The impervious coating inhibits the fibers 12 inthe substrate 10 from becoming oxidized by the atmosphere. Oxidation ofthe fibers 12 in the substrate 10 is undesirable because it tends toweaken the substrate.

The fibers 12 may occupy approximately forty percent (40%) to sixtypercent (60%) of the space defined by the substrate 10. The binder 14can occupy to approximately forty percent (40%) of the space defined bythe substrate 10. Preferably the binder occupies twenty five percent(25%) or less of the space defined by the substrate 10. The mixture ofthe particles 20 can occupy all or any portion of the remaining space inthe substrate 10.

To form the composite material constituting this invention, the binder14 can be applied to a fabric which is woven or non-woven and definesthe substrate 10. This may be provided by any of the methods discussedabove or by dipping the fabric into a liquid constituting the binder.Any excess of binder can then be removed by rolling such excess binderfrom the fabric. The application of a desired amount of the binder tothe substrate 10 can also be controlled by regulating the fluidity orviscosity of the binder. During the above application, the particles maybe suspended in the liquid constituting the binder to permitcoapplication of binder and particles to the fabric.

The fabric defining the substrate 10 may thereafter be disposed in afixture and the substrate may be subjected to pressure and heated tocure the binder. When the binder is cured, it becomes a solid and holdsin place the fabric defining the substrate 10. The magnitudes of theheat and the pressure applied to the substrate 10 are dependent upon thebinder being used. For example, when the binder constitutes pitch, thesubstrate 10 may be heated to a temperature of approximately 850° F. Thesubstrate 10 may be heated to a temperature of approximately 350° F.when the binder is a phenolic resin. When the binder 14 is an acrylic,the substrate 10 may be heated to a melting temperature well known forsuch acrylics and may then be cooled to a temperature of approximately150° F. to prevent the binder from becoming loose.

After the binder 14 has been applied to the fibers 12 defining thesubstrate 10, the substrate is made dense by the application ofpressure. The binder 14 may be then decomposed or pyrolyzed by heatingthe substrate 14 to a temperature in the range of 1000° F. to 5000° F.The particular temperature is dependent upon the specific binder beingused and upon the application to be made of the refractory compositematerial constituting this invention. One of the reasons for decomposingthe binder as by the application of heat is that the binder is notthermally stable, particularly at the elevated temperatures at which therefractory composite material is to be operated. Thus, the binder willtend to decompose at temperatures at, or below, the range oftemperatures in which the refractory composite material may be operated.The decomposition of the binder during the use of the refractorycomposite material is undesirable because it may specifically reduce thelife of the refractory composite material and the operatingcharacteristics of the material during such shortened life. For example,the decomposition of the binder may tend to decrease the strength andhardness of the refractory composite material.

Another binder is preferably applied to the substrate 10 after thebinder previously applied to the binder has been decomposed orpyrolyzed. This binder is applied in any suitable manner such as bychemical vapor deposition or in a liquid form. This binder may also bepyrolyzed or decomposed in a manner similar to that described above. Thesequential application of the binder and the subsequent pyrolyzation ofthe binder through more than one cycle are desirable because theinterstices between the fiber 12 tend to become filled by the pyrolyzedmaterial.

The mixture of the particles 20 has then become locked mechanically inthe pores of the fibers. The disposition of the particles 20 inmechanically locked relationship with the substrate 10 is advantageousbecause it increases the strength of the refractory composite materialwhich is produced. This results in part from the fact that the fibers 12are inhibited by the particles 20 from shearing even when large shearingforces are applied to the refractory composite material. As a finalstep, an impermeable coating 22 is applied to the substrate 10 as bychemical vapor deposition, physical vapor deposition or particle spray(plasma or flume).

FIG. 3 illustrates an article generally indicated at 30 and constitutingone embodiment formed from the refractory composite material of thisinvention. The article 30 includes the pyrolyzed carbon 16. FIG. 4illustrates an article generally indicated at 40 and constitutinganother embodiment formed from the refractory composite material of thisinvention. The article 40 does not include the pyrolyzed carbon 16.

When the refractory composite material of this invention is used in anapplication such as a jet engine or a rocket, the impervious layer 22tends to crack microscopically. This may be considered to be undesirablesince it tends to expose the substrate 10 to the atmosphere. As aresult, the substrate 10 and the pyrolyzed carbon 16 tend to oxidize.The oxidation is accelerated because the refractory composite materialis operating at elevated temperatures. As the substrate 10 and thepyrolyzed carbon 16 oxidize, they tend to vaporize. This causes theimpervious layer 22 to continue to crack microscopically. If anything,the formation of the microscopic cracks in the impervious layer 22 isaccelerated at the elevated temperatures.

At the elevated temperatures, the boron in the mixture of the particles20 tends to oxidize to boron oxide and the boron oxide melts and flows.Boron oxide is relatively non-viscous so that it flows relativelyeasily. The boron oxide tends to flow to the microscopic cracks in theimpervious layer 22 and to seal the cracks. This inhibits any oxidationof the substrate 10 and the pyrolyzed carbon 16.

The silicon in the mixture of the particles 20 also tends to oxidize tosilicon dioxide at the elevated temperatures. The silicon dioxide tendsto flow when it melts. The flow of the silicon dioxide also occurstoward the microscopic cracks in the impervious layer 22 to fill thecracks and inhibit oxidation of the substrate 10. However, silicondioxide is more viscous than boron oxide. Furthermore, the meltingtemperature of silicon dioxide is higher than that of boron oxide. As aresult, the silicon dioxide in the mixture tends to control the rate atwhich the boron oxide flows. Thus, by controlling the relativepercentages of the silicon and boron in the mixture of the particles 20,the effective temperature of the mixture in sealing the microscopiccracks in the impervious layer 22 can also be controlled.

The refractory material such as zirconium oxidizes at elevatedtemperatures in excess of 3000° F., mixes with the silicon dioxide andboron oxide and melts at these elevated temperatures. Zirconium oxide isquite viscous. Thus, the refractory material such as zirconium tends toregulate the rate at which the mixture in the particles 20 can melt andflow into the microscopic cracks in the impervious layer 22.

The particles 20 tend to melt only at the surface of the substrate 10.As a result, the particles 20 continue to provide a mechanical bondingwith the substrate in the remaining area of the substrate. Furthermore,as the refractory composite material of this invention cools afteroperation of the article in which it is incorporated, the microscopiccracks in the impervious layer tend to decrease in size. This causessome of the oxides produced from the mixture in the particles 20 to flowfrom the microscopic cracks into voids in the body of the substrate. Asa result, although the melted oxides fill the microscopic cracks in theimpervious layer 22, they allow the microscopic cracks to breathe sothat the cracks can decrease in size as the temperature decreases.

Although this invention has been disclosed and illustrated withreference to particular embodiments, the principles involved aresusceptible for use in numerous other embodiments which will be apparentto persons skilled in the art. The invention is, therefore, to belimited only as indicated by the scope of the appended claims.

I claim:
 1. In a refractory composite material,a substrate formed from aplurality of fibers of a carbonaceous material, the fibers beingdisposed in a porous non-woven relationship, pyrolyzed carbon materialdisposed on the fibers of the substrate, and particles of a mixutre of aboride, a silicon compound and a refractory material mechanically lockedin the pores of the substrate.
 2. In the refractory composite materialset forth in claim 1,an impervious coating disposed on the substrate. 3.In the combination set forth in claim 1,the boride having a volume rangein the mixture of approximately sixty five (65) parts to ninety five(95) parts, the silicide having a volume range in the mixture ofapproximately one (1) part to twenty one (21) parts and the refractorycomponent having a volume range in the mixture of approximately one (1)part to twenty one (21) parts.
 4. In the combination as set forth inclaim 1,the boride having a volume range in the mixture of approximatelyfifty five (55) parts to eighty five parts, the silicide having a volumerange in the mixture of five (5) parts to twenty five (25) parts and therefractory component having a volume range in the mixture ofapproximately seven (7) parts to twenty seven (27) parts.
 5. Incombination for operation at a particular temperature,a substrate formedfrom a plurality of fibers defining a porous relationship, the fibershaving a particular diameter in the micron range, and a plurality ofparticles having a thickness in the order of the fiber diameter anddisposed in the pores of the substrate in mechanically lockedrelationship with the fibers defining the substrate, the particlesincluding a first relatively non-viscous material having properties ofoxidizing and melting at temperatures considerably less than theparticular temperature and further including a refractory materialhaving properties of oxidizing and having an oxide melting temperaturegreater than the melting temperature of the oxide of the first material,the refractory material being relatively viscous.
 6. In a combination asset forth in claim 5,the particles including a third material havingproperties of oxidizing and having an oxide melting temperature higherthan the melting temperature of the oxide of the first material andlower than the oxide melting temperature of the refractory material, theviscosity of the oxide of the third material being greater than that ofthe oxide of the first material and less than that of the oxide of thesecond material.
 7. In a combination as set forth in claim 6,a pyrolyzedcarbon covering the fibers.
 8. In a combination as set forth in claim 5,an impervious material covering the substrate.
 9. In a combination asset forth in claim 8, the fibers being made from a carbonaceousmaterial.
 10. In combination,a porous substrate formed from a pluralityof fibers having a porous relationship, the fibers having a particularthickness, pyrolyzed particles disposed on the fibers defining theporous substrate, and a mixture of particles mechanically locked in theporous substrate, the mechanically locked particles being formed from amixture of the following: (a) a material selected from group 3a of theperiodic table, (b) a material selected from group 4a of the periodictable, and (c) a refractory material selected from groups 4b and 5b ofthe periodic table.
 11. In a combination as set forth in claim 10,thematerial selected from group 3a of the periodic table having apercentage greater than fifty percent (50%) by volume in the mixture.12. In a combination as set forth in claim 10,an impervious layercovering the porous substrate.
 13. In a combination as set forth inclaims 11,an impermeable layer covering the porous substrate, theimpermeable layer being formed from a material selected from the groupconsisting of the carbides and nitrides of silicon, titanium andhafnium.
 14. In combination for operation at elevated temperatures,asubstrate defined by fibers disposed in a porous relationship to definea porous substrate, the fibers occupying approximately forty percent(40%) to sixty percent (60%) of the space defined by the substrate,pyrolytic carbon disposed on the fibers and occupying to approximatelytwenty five percent (25%) of the space defined by the substrate, and amixture of particles occupying at least a portion of the remaining spacedefined by the substrate, the mixture of particles having approximatelythe same dimensions as the diameter of the fibers, the mixture ofparticles including a first material having properties of oxidizing andhaving a relatively low oxide melting temperature, a second materialhaving properties of oxidizing and having an increased oxide meltingtemperature and a refractory material having properties of oxidizing andhaving an oxide with an oxide melting temperature considerably above themelting temperatures of the first and second oxide materials, theparticles in the mixture being mechanically locked in the pores in thesubstrate, the oxide of the first material being non-viscous, the oxideof the second material being more viscous than the oxide of the firstmaterial and the oxide of the third material being considerably moreviscous than the oxides of the first and second materials.
 15. In acombination as set forth in claim 14,an impermeable layer coating thesubstrate, the impermeable layer having properties of producingmicroscopic cracks at the elevated temperatures and the mixture havingproperties of melting at temperatures below such elevated temperatureand of flowing into such cracks.
 16. In a combination as set forth inclaim 14,the particles in the mixture having a size in the order of thediameter of the fibers and the fibers having a diameter in the micronrange.
 17. In a combination as set forth in claim 15,the fibers having adiameter in the order of seven (7) to eight microns (8μ) and theparticles in the mixture having a size in the order of five microns(5μ).
 18. In a combination as set forth in claim 17 whereinthe fibersare carbonaceous and the first material in the mixture is selected fromgroup 3a of the periodic table, the second material is obtained fromgroup 4a of the periodic table and the refractory material is selectedfrom groups 4b and 5b of the periodic table.